The present invention relates to an ignition system, and more particularly to a laser-induced ignition system with a cavity.
It is desirable to burn lean mixtures in spark ignition engines to improve both fuel economy and emission characteristics. When mixtures which are lean or diluted with exhaust gases are used, the ignition system has a critical influence on misfire or cycle-to-cycle variation. Similarly, ignition is an important design factor in gas turbines, rocket combustors, and the like.
Various ignition systems including high energy spark plugs, plasma jet ignitors, rail plug ignitors, laser-induced ignition, flame jet ignitors, torch jet ignitors, pulsed-jet combustion, and exhaust gas recirculation ignition systems have been proposed. Among these, an ignition system using an energy source from a laser is utilized.
A laser-induced spark ignition system focuses a laser beam to generate a gaseous breakdown and sufficient laser energy can ignite a fuel/oxidizer premixture. This system has many potential advantages, even though some limitations still exist. For example, a laser-induced spark is a reasonable point energy source in which the amount of energy, the rate of its deposition, and ignition timing can be controlled. It also permits choice of the optimal ignition location, which is not easy in conventional ignition systems. In addition, the absence of a material surface in the vicinity of ignition location minimizes the effect of heat loss during flame kernel development.
There are four basic mechanisms, depending on the mode of energy deposition, by which a laser can produce an ignition kernel; thermal heating, resonant and nonresonant breakdown, and photo-dissociation. The relative importance of each mechanism depends on the wavelength of the laser beam. Nonresonant breakdown is the most frequently adopted ignition mode and is generally termed laser-induced spark ignition.
One of the disadvantages of laser-induced spark ignition is that only a portion of laser energy is absorbed by gaseous medium in the vicinity of the ignition location. The rest of the laser energy, for example ranging from 30 to 70%, is lost since the unabsorbed laser beam passes through the ignition location, so that it cannot be utilized in the ignition process.